In general, most consumers think that a case for a wristwatch is preferably made of metal. Such consumers' taste is often found in people in the middle to high age groups in terms of generation, and in Japan and Europe in terms of region.
For a radio wave wristwatch with a built-in antenna, a radio wave permeating material such as plastics and ceramics are often used as a material for the case. If a metal such as stainless steel is used as the material for the case, radio wave time signals cannot sufficiently penetrate the case, so a normal operation for receiving radio waves cannot be anticipated. If a plastic material is used as the material for the case, purchasers of such radio wave wristwatches with a built-in antenna tend to have a bias toward younger generations mainly because of insufficient sense of luxury in appearance, and the wristwatches of this type actually have poor sales among people in the middle to high age groups. Also, if ceramics is used as the material for the case, sales diminishes due to a high sales price.
On the other hand, in a radio wave wristwatch with an externally mounted antenna, having a receiving antenna disposed on the outside of the case, by accommodating the receiving antenna within a specially designed plastic case, or incorporating it in a leather watchband, the wristwatch case itself containing a movement can be made of metal. However, such radio wave wristwatch with an externally mounted antenna lacks simplicity and smartness in terms of appearance, and the connecting construction between the antenna and the movement becomes complicated, and thereby its sales are not widely spread.
Accordingly, the inventors conducted studies earnestly to develop a radio wave wristwatch with a built-in antenna, capable of receiving radio wave time signals with high sensitivity while using a metal case that is less permeable to radio waves. The process reaching the development is as described below.
FIG. 6 is a sectional view showing a construction of a conventional metal wristwatch case used for an ordinary wristwatch (ordinary wristwatch other than the radio wave wristwatch).
As shown in FIG. 6, this metal wristwatch case 101 is constructed by integrally combining three elements in a lapped manner: the three elements are, a metal annular base 102 for shaping a case contour, a metal annular window frame (generally, referred to as a “bezel” also) 104 positioned on the surface side of the metal annular base 102 to fringe a transparent window plate 103, and a metal back cover 105 positioned on the back surface side of the metal annular base 102.
Inside the metal wristwatch case 101, a metal dial plate 106 is disposed so as to face the transparent window plate 103, and a movement 107 is contained in a space 108 between the metal dial plate 106 and the metal back cover 105.
As the metal annular base 102 shown in the figure, for example, a material SUS304 having a thickness of 4 mm, an inside diameter of 32 mm, and a height D1 of 6 mm is used. Also, for example, a glass material having a thickness of 1 mm and a diameter of 30 mm is used as the transparent window plate 103. Also, for example, a material SUS304 having a thickness of 4 mm, an inside diameter of 28 mm, and a height of 3 mm is used as the metal annular window frame 104. Also, for example, a material SUS304 having a thickness of 2 mm and a diameter of 36 mm is used as the metal back cover 105. Also, for example, a material Bs having a thickness of 0.6 mm and a diameter of 30 mm is used as the metal dial plate 106. Further, for example, an ordinary wristwatch movement having a diameter of 26 mm and a height of 5 mm is used as the movement 107.
If the ordinary wristwatch movement 107 is merely replaced with a movement with a built-in antenna for radio wave wristwatch on the basis of the construction shown in FIG. 6, the movement for the radio wave wristwatch hardly receives radio wave time signals. The reason for this is that the movement for the radio wave wristwatch is completely enclosed by the metal dial plate 106 on the upper side, the metal back cover 105 on the lower side, and the relatively thick metal annular base 102 on the periphery.
Thereupon, the inventors made an attempt to use a dial plate made of a radio wave permeating material (for example, glass or plastics) replacing the metal dial plate 106, and to use a back cover made of a radio wave permeating material (for example, glass or plastics) replacing the metal back cover 105. Thereby, the radio wave time signal reception sensitivity was improved considerably due to the removal of radio wave shielding elements positioned above and below the movement. However, the radio wave time signal reception sensitivity did not reach a level for practical use.
Next, the inventors paid attention to the radio wave shielding elements positioned at the side of the movement 107, and attempted to decrease the thickness of the metal annular base 102. However, since the metal annular base 102 must maintain the strength of the case and hold an operating push button penetrating the metal annular base 102, the decrease in thickness thereof has a limitation. Therefore, although the radio wave time signal reception sensitivity was improved to some degree, it did not yet reach a level for practical use.
The present invention has been made in view of the above-described technical background, and accordingly an object thereof is to realize a radio wave wristwatch having a metallic appearance like the appearance of an ordinary wristwatch, by improving radio wave permeability in the vertical direction and at the side of a movement while the strength and thickness of a metal annular base constituting a case body is kept at a required level.
Another object of the present invention is to provide a radio wave wristwatch having a metallic appearance that can also be manufactured at a low cost.
Other objects and advantages of the present invention will become apparent for a person skilled in the art from the following description.